The present invention has particular application to an aqueous, reverse-flow cooling system for an internal combustion engine. The system constitutes a modification of the non-aqueous reverse flow system disclosed in my U.S. Pat. No. 4,550,694 that renders use of an aqueous coolant practical.
U.S. Pat. No. 4,550,694 teaches that in most conditions of operation of an internal combustion engine, coolant boiling takes place around the combustion chamber area typically termed the cylinder head cooling chamber. Coolant vapor produced by such boiling must be accommodated and said patent discloses one method comprising a particular fluid circuit that is suitable for a non-aqueous coolant.
However, if a reverse flow cooling system is operated with an aqueous coolant, for example, a 50/50 ethylene glycol/water solution, the lower molar heat thereof as compared to, for example, non-aqueous propylene glycol, produces considerably more water vapor than the propylene glycol vapor for the same heat load. The increased vapor volume may become trapped in the cylinder head cooling chamber, ultimately displacing coolant in the cylinder head cooling chamber until the pressure of the vapor exceeds the setting of the relief valve. Coolant loss is exhibited during the open phase of the relief valve.
This alternating cycle of building vapor volume and subsequent venting results first in too much coolant vapor in the cylinder head coolant chamber, which is evidenced by "knock," and then venting of coolant vapor resulting in excessive coolant loss.
More particularly, while the existence of noncondensible air and leak-induced combustion gases in the cooling chambers of an engine has long been recognized, the existence and effect of excessive water and coolant vapor in the cooling chamber of an engine has long been recognized, the existence and effect of excessive water and coolant vapor in the cooling chambers of an engine utilizing an aqueous coolant has only recently been recognized. Many of the problems attributed to noncondensible gases are actually caused by water vapor and vaporized coolant. The basic assumption was that water and coolant vapor was generated only when coolant flow ceased during an "after-boil" condition incident to engine shut down. It was further assumed that small amounts of water and coolant vapor, if generated while the engine is running, will move out of the engine with coolant flow into the radiator where it condenses into a liquid.
However, since coolant vapor exists in the cylinder head cooling chambers at all engine loads and conditions, most of the debilitating effects of noncondensible gases, specifically air and combustion gases, are also exhibited by coolant vapor generated at such normal engine loads and conditions.
To understand the engine dynamic that exists in the liquid cooled internal combustion engine due to the physical characteristics of aqueous coolants and the mechanical structure of the liquid cooling system, the characteristics of coolant vapor must be understood.
Although the currently used aqueous coolant, normally one-half ethylene glycol and one-half water (50/50 EGW), exhibits a very low freezing point compared to water, the boiling and condensation characteristics of the solution remain close to those for water. Within the engine cooling system there exists a range in the saturation temperature, or in other words the boiling point and maximum condensation temperature of pure water, from a high, at I atmosphere gauge, of 121.degree. C. (250.degree. F.) down to, at zero psig., 100.degree. C. (212.degree. F.). Because water exhibits a relatively high vapor pressure compared to ethylene glycol, when a 50/50 EGW mixture is boiled, the resultant vapor is more than 98% water vapor by volume. This water vapor will not condense in the coolant when the coolant temperature is above the saturation temperature of water for the system pressure. Ideally, in the conventional 50/50 EGW cooling system, water vapor generated by boiling in the cooling chambers would be condensed into the liquid coolant itself. However, under high load and/or high ambient temperature conditions, when the coolant temperature approaches the saturation temperature of water at a given system pressure, vapor in the cooling jacket cannot condense soon enough to prevent the displacement of coolant. As long as the 50/50 EGW bulk coolant temperature is above the saturation temperature of the water vapor fraction of the coolant, the water vapor fraction suspended in the coolant will persist and accumulate even though it is below the boiling point of the 50/50 EGW mixture. This means that even if the area generating the water vapor is small i.e., a single hot spot such as an exhaust valve seat, and the bulk temperature of the coolant is above the saturation temperature of water at a given system pressure, the water vapor fraction will continue to grow, if not addressed, until system failure occurs. Failure of the conventional 50/50 EGW cooling system is usually in the form of vapor binding of the coolant pump, evidenced by loss of flow, vapor binding of the cylinder head evidenced by severe engine knock, or catastrophic venting evidenced by boil-over.
It is also to be noted that vapor conditions inside the engine cooling chambers are different than conditions in other areas of the liquid cooling system. Large volumes of vapor can be suddenly released from the cooling chambers merely by a reduction in engine RPM after a sustained steady state condition i.e., highway driving. This condition is the result of the coolant pump generating a relatively high system pressure within the coolant chambers. Depending upon coolant flow, which is a function of pump speed, the additional mechanical pressure placed upon the coolant over the bulk system pressure can be as high as 35 to 80 psig. The result is that considerably more coolant vapor is generated at lower than expected system bulk temperature and pressures.
For example, on a moderately warm ambient day, testing has shown that a conventional 50/50 EGW system can stabilize at an engine outlet coolant temperature of 115.degree. C. (240.degree. F.) resulting in a radiator outlet temperature of about 111.degree. C. (232.degree. F.) at a system pressure of approximately 7 psig. The relatively low system pressure is a result of the bulk temperature being below the 50/50 EGW saturation temperature of about 118.degree. C. (245.degree. F.). Although the average temperature in the engine, in theory, would be 113.degree. C. (236.degree. F.) with a temperature gain of 4.degree. C. (8.degree. F.), there are actually areas in the engine coolant chambers, because of localized high heat flux values and static coolant flow areas, which exceed the saturation temperature of 118.degree. C. (245.degree. F.), of the 50/50 EGW at 7 psig. However, because of the mechanical pressure exerted by the pump on coolant within the engine coolant chambers, coolant pressure in said chamber is raised by as much as an additional 35 to 80 psig. Therefore, the saturation temperature of the 50/50 EGW, in the engine coolant chambers, is raised to a point where no vapor is generated. This condition remains stable, with no vapor produced within the engine coolant chambers until the RPM of the engine, and therefore pump speed, are lowered. Immediately upon lowering of the pump speed, the mechanical pressure placed upon the coolant is lowered, as is the resultant saturation temperature of the 50/50 EGW coolant within the engine cooling chambers. Instantaneously vapor is generated at all high heat flux areas and regions of static coolant flow which were only momentarily before kept free of vapor by the increased pressure induced by the higher engine/pump speed. The water vapor fraction produced cannot be condensed as the lowest system coolant temperature is above the condensation temperature of the water vapor or, more specifically, the water vapor generated in the cylinder head cooling chamber is produced at a 50/50 EGW saturation temperature of 118.degree. C. (245.degree. F.) at 7 psig. and the condensation temperature for the water vapor fraction is 111.degree. C. (232.degree. F.) at 7 psig. Therefore, the only way for water vapor, which remains as vapor at the bulk coolant temperature of 115.degree. C. (240.degree. F.) of the system, to be removed from the top of the engine chambers, in current engines utilizing conventional coolant flow direction, is for the water vapor to be carried out of the upper chamber area, to the radiator, continually, until the system temperature is lowered below 115.degree. C. (240.degree. F.). In the engine off load condition, the system temperature must be lowered, below 118.degree. C. (245.degree. F.) in order to effect condensation even though the radiator outlet temperature is low enough, for example, 111.degree. C. (232.degree. F.), to start condensing the water vapor, not taking into account that the pressure depression of the pump draw would actually lower the required condensation temperature below 111.degree. C. (232.degree. F.) at that point.
If coolant flow is reversed in a currently mass produced engine so as to flow from the outlet side of the pump to the engine cylinder head cooling chamber, down through the internal passages in the engine block into the lower cylinder block cooling chamber, and thence out of the lower chamber to the inlet side of the pump, the system would work only if cooling jacket vapors and noncondensible gases are totally eliminated. This is contrary to the reality of liquid cooled engine operation. In reality, reversal of coolant flow aggravates problems associated with water and coolant vapor formed when the coolant temperature inhibits condensation. Because of, primarily, the force of the reverse flow coolant entering high into the cylinder head cooling chamber, such vapors and noncondensible gases cannot exit the chamber while the engine is running, since the flow of coolant downwardly through passages in the block tends to bias the gases toward the block cooling jacket. Moreover, secondly, in the mechanical sense, coolant velocity through the block passages is insufficient for the coolant to carry the gases completely downward. Stated in another manner, natural buoyancy of the gases will overcome the downward coolant flow and the gases will rise into the upper portion of the cylinder head coolant chamber progressively displacing more and more coolant as the vapor fraction of the chamber increases.
After a period of time, a significant and potentially damaging accumulation of trapped vapor and other gases may occur in the cylinder head cooling chamber. For example, 10 grams (0.35 oz) of water, vaporized in the cylinder head cooling chamber produces 12.3 liters (3.25 gallons) of vapor. Without a means to remove the uncondensed vapors from the cylinder head jacket as they enter the chamber or are generated within the jacket, a steady and oftentimes rapid displacement of coolant occurs. As the displacement of coolant from the cylinder head cooling chamber continues, due to the water and coolant vapor fractions expanding and backing liquid coolant out into the radiator, pressure rises at the vent, typically found at the top of the radiator, and coolant is released from the system into the atmosphere when the vent setting is exceeded. As the generation of gases in the cylinder head cooling chamber continues, displacement of coolant causes the gaseous fraction of the chamber to increase until the loss of coolant volume progresses to a level where heat exchange values of metal to coolant, are imbalanced and excessive boiling occurs. The boiling rapidly passes from a nucleate boil condition to a film boiling state and finally to a superheated boiling condition wherein a major portion of the cylinder head cooling chamber is filled with vaporized coolant and violent venting of the system occurs. If the engine is not shut off during such a period of "vapor-binding" of the cylinder head cooling chamber, engine damage may result.
The conventional cooling system found in today's engines, ameliorates the aforesaid situation by utilizing upward flow of coolant from the engine block coolant chamber to the cylinder head coolant chamber. Thus, the natural buoyancy of the coolant vapor helps remove surface vapor by "lifting and scrubbing" the vapor off the surface of the coolant chambers. The "lift and scrub" action tends to maintain the surface vapor condition in a reasonably controlled state, notwithstanding the fact that vapor generated by aqueous coolants has a relatively high surface tension characteristic, for example, from 56 to 70 dynes/cm at 25.degree. C. and tends to "cling," evidencing nucleate or film boiling, to the metal surface of the cylinder head coolant chamber.
The aforesaid "saving" feature of conventional coolant flow direction does not obtain when the coolant flow is reversed whereby coolant enters the cylinder head coolant chamber and flows downwardly into the cylinder block coolant chamber exerting a downward pressure against the natural buoyancy of the coolant vapor and other gases, decreasing the tendency of the vapor to break loose from the metal surfaces of the coolant chambers.